Regret can be a poison, sneaking into the system and overtaking any other rational emotions or thoughts. And it'd be plausible to think USC quarterback Mitch Mustain is battling a pretty severe case of regret.

He was the starting quarterback for a winning team in the lauded SEC, undefeated as a starter in his freshman year. Instead of staying at Arkansas, his home state school, Mustain packed up and moved his life all the way to the West Coast and USC. He sat out for a year while John David Booty led USC to the Rose Bowl.

The following spring, Mustain competed for the No. 1 spot with Mark Sanchez, and he lost. With Sanchez leaving for the NFL, the door opened up again for Mustain, but after this spring, he's now the No. 3 quarterback, backing up two players with more eligibility than he has - redshirt sophomore Aaron Corp and true freshman Matt Barkley.

So when it was announced that Corp had won the starting job this spring, people were surprised when Mustain definitively said he wouldn't transfer for a second time.

A local radio host took Mustain to task for transferring to USC in the first place and for not transferring twice - at the end of spring 2008 or at the end of this most recent spring ball.

It's hard not to wonder why Mustain would do this to himself. It seems like a lifetime ago when Mustain was leading Arkansas to a touchdown against the Trojans in his first drive as a college quarterback.

That drive and the subsequent eight wins with Mustain as the starter started what Mustain on a path to college stardom, but after two quarterback battles at USC, the path now seems to be leading elsewhere.

If the best-case scenario for the Mustain who led Arkansas to a SEC West title was he'd end up like Joe Montana, it's now Matt Cassel.

While there's now a precedent for a USC backup quarterback to succeed in the NFL, there hasn't been a third-string quarterback to take his game to the next level - and that's what Mustain is.

Making things even tougher on him, Mustain has to watch his former high school teammate and Arkansas teammate Damian Williams prosper in USC's system while he flounders.

If his future at USC is going to still bright, and there is a chance, Mustain has to play with the same passion and skill that he showed on his way to becoming one of the most decorated recruits in the 2006 class.

He needs to climb two rungs on the ladder, which won't be easy. He's got to outplay Corp and Barkley all summer and all of fall camp. And, he needs some luck - maybe an injury or two.

If he can make good on the promise he showed back during his freshman season, Mustain won't have to answer any more quest ions about whether he made the right decision or not. And he won't have to fight any more battles with regret.

If not, Mustain will always have to wrestle with what could have been, and that could be his biggest challenge yet.